1. Field of the Invention
The present invention relates to a polyester resin for a dry toner which is used for developing an electrostatic image in an electrophotographic process, an elctrostatically recording process, an electrostatically printing process or the like. More particularly, the invention relates to a polyester resin for a dry toner having a high resistance to off-setting and excellent grinding properties.
2. Description of the Related Art
In a method of obtaining a permanent real image from an electrostatic image, an electrostatic image formed on a photoconductive photosensitive material or an electrostatically recording material is developed with a toner, which has been charged by friction in advance, and then fixed.
The fixing is carried out either by directly fusing the toner image obtained by the development on a photoconductive photosensitive material or an electrostatically recording material, or by transferring the toner onto a paper or film, and then fusing it onto the transferred sheet. The fusion of the toner image is carried out by bringing it into contact with a solvent vapor, applying pressure, and heating. The heating is carried out by a non-contact heating method using an electric oven or a roll-bonding method using heated rolls; currently the latter is chiefly used because of the requirement for an accelerated fixing stage.
A one-component type toner or a two-component type toner is used in the dry developing process. The two-component toner is produced by initially melt-kneading a resin, a coloring agent, an electrostatic regulator and other required additives, to sufficiently disperse them, and then coarsely grinding and finely grinding the mixture to a predetermined particle size classification. The one component type is produced similarly but with the addition of a magnetic iron powder to the respective components of the two component type toner.
Because it is the main component in the toner formulation, the resin has a great influence over the major properties for the toner, and accordingly, the resin used for a toner must have a good dispersibility of the coloring agent in the melt kneading stage and good grindability in the grinding stage, during the toner production. Furthermore, a wide variety of properties such as a good fixing, good off-setting, good blocking, and good electric properties are required for the toner, when used.
As the resins used in the production of the toner, epoxy resins, polyester resins, polystyrene resins, methacrylic resins and the like are known. In the case of the combined process of crimping, heating, and fixing, a copolymer of styrene and a (meth)acrylate has been chiefly used, but due to its capability of being fixed at a lower temperature and the high resistance of the toner image, which has been fixed, to a plasticizer for the vinyl chloride resin, a polyester resin is now increasingly used.
Polyester resins are produced by a direct esterification of a divalent carboxylic acid or a lower ester thereof and a diol, or by the condensation reaction in the ester-exchange. In the polyesters for a toner, to impart an off-setting resistance in the fixing stage, it has been suggested that a tri- or poly-valent carboxylic acid or alcohol be copolymerized with the above-mentioned usual components, to provide a weakly cross-linked structure.
However, a toner which uses the conventional polyester resin obtained by the copolycondensation of a tri- or poly-valent carboxylic acid or alcohol has a disadvantage of a poor grinding efficiency during the toner production, and this disadvantage must be alleviated.
The inventors have investigated the causes of why a toner having an excellent resistance to off-setting can be achieved by a copolycondensation of the tri- or poly-functional monomer in the prior art, but the efficiency of grinding during the toner production is worsened and as a result, found that the grinding efficiency can be kept within a satisfactory range by selecting the proper ratio of a tri- or poly-valent acid or alcohol monomer to an aromatic diol or an alicyclic diol. That is, the following findings have been made. If the tri- or poly-functional monomer is used in a large amount, the molecular weight M.sub.w is lowered and although this leads to an increased grinding speed, a large amount of fine powder which cannot be used for toner is generated, and thus this leads to a poor yield and a lower productivity. Conversely, if the amount of the tri- or poly-functional monomer is small, although the molecular weight M.sub.w is increased, the grinding speed is markedly lowered, resulting in a lowering of the toner productivity.